The present invention relates to vacuum pumps of the gas transfer type. In particular, but not exclusively, the present invention relates to a new type of drag vacuum pump mechanism.
In general, vacuum pumps can be split into various categories according to their pumping mechanism. Thus, in broad terms, a vacuum pump can be categorized as either a gas transfer pump or an entrapment pump. Gas transfer pumps can be further classified as kinetic pumps or positive displacement pumps (which includes reciprocating pumps and rotary displacement pumps such as Roots or rotary vane mechanisms). Kinetic pumps can also be further classified as drag pumps (such as molecular drag pumps or turbo-molecular pumps) or fluid entrainment pumps (such as oil vapour diffusion pumps).
In order to achieve a certain level of vacuum pressure, different types of pumps can be arranged to operate in series in order to compress low pressure gases to pressures at or just above atmospheric pressure. The different classification of pumps used in such a pumping arrangement depends on many factors, including the level of vacuum pressure required, the application requiring a vacuum environment, the volume of material to be pumped within a certain timeframe and the material being pumped through the vacuum pump, for instance.
Gas transfer vacuum pumps are currently used in many different industrial and scientific applications. For instance, gas transfer pumps provide vacuum for the manufacture of semiconductor devices, including, but not limited to the manufacture of integrated circuits, microprocessors, light emitting diodes, flat panel display and solar panels. These applications require a relatively sterile or benign environment in order to enable deposition and processing of material on a substrate. In addition, gas transfer pumps are used in other industrial processes that require vacuum, including glass coating, steel manufacture, power generation, vacuum distillation, lithium ion battery production and the like. Some scientific instruments, such as mass spectrometers or electron beam microscopes, also require vacuum environments and gas transfer pumps are often used to achieve a suitable vacuum environment.
Various types of gas transfer pump mechanisms have been developed over time. Different pump mechanisms were developed according to the requirements of the application and as a result of different flow behaviour of gas molecules at different vacuum pressures. For instance, at high vacuum pressures (10−3 mbar and below) the gas molecules are said to be in a molecular flow regime. Here, the molecules move freely without mutual hindrance and collisions are mainly with the walls of a vessel. Molecules strike the vessel's wall, stick for a relatively short period, and then leave the wall's surface in a new and unpredictable direction. The flow of gas is random and the mean free path is relatively large. In molecular flow regimes, pumping occurs when molecules migrate into the vacuum pump of their own accord. At vacuum pressure in the region of atmospheric pressure to about 1 mbar, the gas molecules behave in a different manner Δt these higher pressures, the flow is called viscous flow. Here the gas molecules collide with one another frequently and the mean free path of the molecules is relatively short. Turbulent and laminar flow conditions exist in this pressure regime. The pressure regime between molecular and viscous conditions is termed transitional flow regime (from about 1 mbar to 10−3 mbar).
However, there is no known single type of pump mechanism that can operate at required high efficiency across all the vacuum pressure regimes. Thus, in order to evacuate a chamber to a high level of vacuum pressure (10−6 mbar, for instance), a vacuum pump system might include a turbo-molecular pump (which are designed to operate efficiently at pressures between 10−9 to 10−2 mbar) backed by a molecular drag pump mechanism (which operate efficiently in the transitional flow regime) and further backed by a scroll, Roots or screw pump (which operate efficiently in the viscous flow regime and exhaust gas at atmospheric pressures), depending on the application requirements.
Certain molecular drag mechanisms were developed in the first half of the 20th Century and subsequently optimized. However, the fundamental arrangement of the various drag mechanism configurations has remained unchanged, save for the developmental design tweaks. In essence, drag pump action is produced by momentum transfer from a relatively fast moving rotor surface directly to gas molecules contained within a channel defined by a stator. The mechanisms have taken the names of their principle developers.
For instance, in the Gaede pump mechanism shown in FIG. 1 (which is named after Wolfgang Gaede 1878-1945) gas molecules are forced to traverse a set of rotating impeller disks 1, each of which is rotating in close proximity to a stationary gas channel 2 whose inlet 3 and outlet 4 are separated by a stationary stripper member 5 that urges molecules away from the rotating disk at the outlet and into the inlet of the next stage (also see patent documents U.S. Pat. No. 852,947 and GB190927457).
The Holweck pump mechanism shown in FIG. 2 generally comprises a smooth sided cylinder 6 spinning in close proximity to a helical grooved outer wall 7 and is named after Fernand Holweck, (1890-1941). The tangential velocity of the cylinder imparts momentum to the gas molecules which are propelled within the grooved channels along the helical path towards an outlet 4. Multiple grooved surfaces are commonly used (reference can be made to U.S. Pat. No. 1,492,846 for more details). In alternative Holweck configurations, the smooth sided cylinder can form the stator and the rotor can be configured as the helical grooved component.
In a Siegbahn pump mechanism, as shown in FIG. 3, the rotor generally comprises a spinning disk 8 to impart momentum to the gas molecules. The stator comprises spiral channels on its surface held close to the rotating disk. Thus, gas molecules are forced to travel along the inwardly spiralling radial channels. This mechanism was developed by Mane Siegbahn (1886-1978) and is further described in patent document GB332879.
A more detailed explanation of these known mechanisms and their additional incarnations is not necessary here because the skilled person is familiar with them. The various mechanisms form part of the common general knowledge of the person skilled in the art of vacuum pump technology, with further explanation found in various books on the subject. For example, reference can be made to the following textbooks: “Modern Vacuum Practice”, by Nigel Harris, published by McGraw-Hill in 2007 (ISBN-10:0-9551501-1-6); “Vacuum Science and Technology”, edited by Paul A Redhead, published for the American Vacuum Society by AIP Press in 1994 (ISBN 1-56396-248-9); and “High-Vacuum Technology—A Practical Guide”, by Mars Hablanian, published by Marcel Dekker Inc in 1990 (ISBN 0-8247-8197-X).
Both Holweck and Siegbahn mechanisms are commonly used as backing pumping mechanisms for turbo-molecular pump mechanisms. Advantageously, the Holweck or Siegbahn rotor can be integrally coupled to the turbo-molecular pump's rotor thereby allowing for a single rotor and drive motor design. Such pump mechanisms are referred to as compound turbo-molecular pumps and examples of this type of pump are disclosed in U.S. Pat. Nos. 8,070,419, 6,422,829 and EP1807627, for example.
However, known molecular drag mechanisms suffer from various drawbacks. For instance, the capacity of the pump mechanism is limited because the rotor has to rotate relatively close to the stator and the depth of the stator channel has to be relatively shallow in order to optimize the compression ratio of the pump. In known drag pump mechanisms, it is not possible to increase capacity by increasing the depth of the stator channel beyond a certain limited. The system being evacuated is at a lower gas pressure than at the pump's exhaust and gas naturally tries to flow back through the pump into the evacuated system to equalize any pressure gradient. If the stator channel is too deep, then gas molecules in a portion of the channel furthest from the rotor can be unaffected by the rotor. Thus, a path for gas molecules to flow back along the channel against the intended flow direction towards the inlet of the drag pump stage can be provided when the channel is too deep resulting in a significant loss of pump efficiency and compression ratio.
There is a desire to increase the capacity of drag mechanism vacuum pumps. This might be achieved by providing several drag mechanisms arranged in a parallel configuration, such as the system disclosed in U.S. Pat. No. 5,893,702. Here, concentric Holweck pump stages are arranged to work in parallel with one another. However, the additional rotor weight, inertia, complexity and overall pump size required by this type of configuration can make it undesirable.
Turbo-molecular pumps comprise a series of rotor blades that extended in a generally radial direction from a rotor axle or hub. A series of rotor blade sets are stacked on top of one another along the axis of rotation. The blades are angled to direct gas molecules struck by the rotating towards an outlet. It is conventional to place stator blades in-between each rotor blade set to improve pump efficiency and reduce backflow of gas molecules towards the pump inlet. The stator blades are generally designed along the same principles as the rotor blades, but the stator blades are angled in an opposite direction. The rotor and stator blades can be machined from a metal block or formed from a sheet of metal having the blades stamped out of the sheet. The skilled person is familiar with this type of vacuum pump and further description of the mechanisms is not necessary here. Alternative turbo-molecular pump designs have been proposed that can be described as radial flow turbo-molecular pumps, such as those shown in US2007081889, U.S. Pat. No. 6,508,631 and DE10004271.
Both axial and radial flow turbo-molecular vacuum pumps are efficient only in the molecular flow regime pressures because the pump relies on high speed rotors imparting momentum to gas molecules and directing the molecules towards the outlet. At higher pressures, that is in the transitional and viscous flow regimes where gas molecules interact with one another as well as part of the pump, turbo-molecular pumps become much less efficient. This reduction in efficiency is manifested as an inability of turbo-molecular pumps to provide an effective compression ratio of gases at relatively low vacuum pressures. In effect, at low vacuum pressures (i.e. in the transitional and viscous flow pressure regimes) the gas molecules can become ‘trapped’ in-between the blades of a rotor or stator as a result of interacting with neighbouring gas molecules rather than the parts of the pump designed to direct molecules towards an outlet. Thus, at these higher pressures, the pump can suffer from a so-called ‘carry over’ where gas molecules are not effectively transferred along the axial length of the pump towards the outlet but tend to remain in the space between neighbouring rotor blades and travel in a generally circumferential path.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.